1. The Field of the Invention
The present invention is related to a fastener installation tool incorporating a novel air return chamber for driving piston return. More particularly, the present invention relates to the use of a air return chamber sealed by a one-way valve which is pressurized during the pull stroke to provide increased force to drive piston return.
2. Technical Background
Pintail fasteners, including swage fasteners and rivets, are commonly used in industries ranging from aircraft and aerospace manufacturing to building construction. Each fastener includes a deformable head disposed about a shaft and a pintail, sometimes referred to as a mandrel or stem, that is detachably secured to the shaft. The fasteners are applied by using a pneudraulic installation tool. The installation tool applies a force to the fastener which deforms the fastener's head by pulling on the pintail tail until the pintail detaches from the shaft. The deformed head traps workpieces between portions of the fastener, thereby fastening the workpieces together.
Some fastener installation tools are powered solely by air pressure which is generally provided by "shop air" at approximately 90 psi. Such installation tools include pistons which convert the air pressure into greatly intensified hydraulic fluid pressure which drives a hydraulic piston to provide the pull forces required to install the fasteners. However, because such tools include integral pressure intensification mechanisms, they are often heavy and awkward to use.
As a result of such disadvantages, installation tools which are powered through pressurized hydraulic and pneumatic lines that connect the tool to a remote pneudraulic power unit have been developed. The hydraulic pressure intensification is performed at the power unit and is supplied to the tool via a hose, thereby greatly reducing the weight of the tool. Indeed, an improved power unit for use in such an application is disclosed in applicant's parent application, United States patent application Ser. No. 08/427,599, filed Apr. 24, 1995 and entitled Pneudraulic Power Unit, which application is incorporated herein by this reference. With such a power unit, multiple tools may be operated.
Tools which have been designed for use with such remote power units suffer from a variety of disadvantages. It is generally desirable that the weight of such tools be reduced to a minimum while utilizing a design which will be durable and provide high-speed operation. Indeed, a problem found in many installation tools (not only those designed for use with a remote power unit) is that the speed at which the tool may be operated is limited to an undesirable extent by the inability to quickly return the hydraulic piston following the installation, or pull, stroke.
In operation, such fastener installation tools utilize high-pressure hydraulic fluid to drive a hydraulic piston, generally referred to herein as the nose assembly piston. The hydraulic fluid fills a forward piston chamber which drives the nose assembly piston rearward, thereby generating the required installation force which is applied to the fastener. After completing the pull stroke, a return piston chamber on the opposite side of the piston must be pressurized to cause the piston to return to its home position. Once the piston has been completely returned, the cycle may be repeated to install another fastener.
The force required to return the hydraulic piston initially must overcome the static friction of the seals which seal the piston to the piston cylinder. Once the barrier of static friction is overcome, thereby initiating movement of the piston, the return force must be sufficient not only to move the piston within the piston cylinder, but also to "pump" the hydraulic fluid in the forward piston chamber into its reservoir or other holding area.
One conventional method for driving the return of the hydraulic piston is simply to apply pressurized shop air to the piston. The disadvantage of this method is that the available pressure (generally about 90 psi), and hence the available force, is generally too low to provide the piston with suitable return speed.
An alternative approach which is commonly utilized in the art is to employ a helical spring within the piston cylinder. This solution requires that the tool be configured with additional length in the rearward direction, thereby increasing both the size and the weight of the tool. Additionally, over time the spring may lose its preload value, wear out, or otherwise fail.
Yet another approach known in the art is to provide a precharged gas volume in the return piston chamber. This approach is problematic in that it requires special fill valving and elaborate sealing methods. Also, maintenance in the form of frequently replenishing the charge is required to ensure that the return piston chamber is always sufficiently charged.
Hydraulic return techniques have also been proposed. Unfortunately, hydraulic return techniques are generally not satisfactory because they require additional hydraulic hose or the implementation of valving integral with the tool, either of which unacceptably increase the cost and weight of the tool.
From the foregoing, it will be appreciated that it would be an advancement in the art to provide a durable, light-weight fastener installation tool which could be driven with a remote power rig.
Indeed, it would be an advancement in the art if such a tool provided for the quick return of the hydraulic piston following its pull stroke, yet required only minimal maintenance, thereby permitting efficient, high-speed operation.
Such a fastener installation tool is disclosed and claimed herein.